Genetics: The Science of Heredity

Chapter 3

Section 1: Gregor Mendel

• A young priest from Europe, became a teacher and cared for hundreds of pea plants

• Became curious about why some of the plants had different physical characteristics, or traits

• He noticed that some plants were short and some were tall

• He also noticed some had yellow seeds while some had green seeds

Mendel continued

• Mendel observed that some pea plants had different traits that their parents

• The passing of traits from parents to offspring is called heredity

• The study of heredity is genetics

Mendel’s Peas

• Mendel chose pea plants to study because many of their traits exist in only two forms: for example, height is either tall or short, not medium

• Mendel developed a way to cross-pollinate, or “cross” pea plants

• He removed pollen from a flower on one plant and brushed it on a flower of a second plant

Mendel’s Experiments

• To study inheritance of traits, Mendel decided to cross plants with opposite forms of a trait…

• For example, he crossed tall plants with short plants to see what the offspring would be

• He always started his experiment with purebred plants

• A purebred plant is one that always produces offspring with the same form of a trait as the parent (purebred short peas always produce short offspring)

First Experiment

• In his first experiment, Mendel crossed purebred tall plants with purebred short plants (“P generation”)

• He called the offspring of the “P generation” the first filial, or the “F1 generation” (Filial means son in Latin)

• To his surprise, all of the F1 generation plants were tall even though one of the parents were short

Second Experiment

• Mendel let the F1 generation plants self-pollinate • These offspring were known as the F2 generation• Mendel was surprised to see the F2 generation were both tall and

short!• This occurred even though all of the F1 parents were tall!• Mendel discovered about ¾ of the plants were tall and ¼ were short

Other Traits

• Mendel also did experiments to check for:Seed shape, seed color, seed coat color, pod shaped,

pod color, and flower position

Dominant and RecessiveAlleles

• Factors that control each trait exist in pairs…one from the female parent and one from the male

• One factor in a pair can hide the other factor

• In the F1 generation, the tall factor hid the short factor

Alleles continued…• Today, factors that control

traits are called genes• Different forms of genes

are called alleles• In Mendel’s work, each

pea plant inherits a combination of two alleles from its parents– Either:

• Two alleles for tall stems• Two alleles for short stems• One of each

Alleles continued…• Individual alleles control

the inheritance of traits• Some alleles are

dominant– A trait that always shows

up in the organism when the allele is present

• Some alleles are recessive– These are masked, or

covered up, when the dominant allele is present

– A trait controlled by a recessive allele will only show up if the organism does not have the dominant trait

Understanding Mendel’s Crosses• Here is how Mendel’s results

work:• The P generation tall plants

(purebred) had two alleles for tall stems

• The purebred short plants had two alleles for short stems

• The F1 generation received one tall and one short allele

• The F1 plants are called hybrids (they have different alleles for the trait)

• All the F1 plants are tall because the dominant allele (tall) covers the recessive allele (short)

Understanding Mendel’s Crosses continued… • When the F1 plants

(hybrids) were crossed with each other, some inherited two dominant alleles (these became tall plants)

• Some inherited one dominant and one recessive allele (these were tall)

• Some inherited two recessive alleles (these were short!)

Symbols in Genetics

• Letters are used to represent traits in today’s world

• A dominant allele is represented by a capital letter (T)

• A recessive allele is represented by a lowercase letter (t)

• TT would mean two dominant alleles

• tt would mean two recessive alleles

• Tt would mean one dominant and one recessive allele

Mendel’s Contribution

• In 1900, three scientists rediscovered Mendel’s work

• Many of Mendel’s discoveries are still used today

• Because of his important work, Gregor Mendel is known as the “Father of Genetics”

Section 2: Probability

• Probability is the likelihood that a particular event will occur

• If you flip a coin… the probability of landing on heads is 50%, and the probability of landing on tails is 50%

Principles of Probability

• There are three ways to express probability

• As a ratio: 1 to 2• As a percent: 50%• As a fraction ½• The laws of probability predict what

MIGHT occur.• If you flip a coin 10 times you might

get 5 heads and 5 tails, or you might get 1 head and 9 tails…

Mendel and Probability• Remember…when Mendel

crossed two hybrid plants (Tt), ¾ were tall and ¼ were short

• Mendel stated that the probability of producing a tall plant was 3 in 4, and the probability of producing a short plant was 1 in 4

• Mendel was the first scientist to recognize that the principle of probability can be used to predict the results of genetic crosses

Punnett Squares• A Punnett square is a chart

that shows possible combinations of alleles that can result from a genetic cross

• They are also used to determine the probability of a particular outcome

• TT = 25%• Tt = 50%• tt = 25%

Phenotypes• An organisms

phenotype is its physical appearance

• Phenotypes are visible traits

• An example: Pea plants can have one of two different phenotypes for stem height: tall or short

These are physical characteristics of something!

Genotypes • A genotype is the genetic makeup or the allele combinations

• TT, Tt, tt• An organism that has two

identical alleles for a trait is homozygous (TT, tt)

• An organism with different alleles for a trait is heterozygous (Tt)

Codominance

• Sometimes, there is no dominant or recessive allele

• When this happens, the offspring inherits both alleles

• For example, if a purebred black cow is crossed with a purebred white cow, the cow will be both black and white.

Section 3: Chromosomes and Inheritance

• In humans, each body cell (skin, blood, liver, etc.) has 46 chromosomes (or 23 pairs)

• Sex cells (sperm and egg) contain 12 chromosomes

• Walter Sutton, an American geneticist discovered this

• He concluded that genes are carried from parents to their offspring on chromosomes.

• This is known as the Chromosome Theory of Inheritance

Meiosis• Meiosis is the process by

which the number of chromosomes is reduced by half to form sex cells (sperm and egg)

• During meiosis the chromosome pairs separate and are distributed to two different cells.

• The sex cells that are created have only half as many chromosomes as the other cells in the organism.

Chromosomes

• In body cells of humans, there are 23 pairs (46 total) chromosomes

• Chromosomes are made of many genes joined together like beads on a string

• Each gene controls a particular trait

Section 4: DNA Connection• The main function of genes is to

control the production of proteins in cells

• Proteins help to determine the size, shape, and other traits

• DNA is a major component in chromosomes

• A = Adenine• T = Thymine• G = Guanine• C = Cytosine• Remember, these four bases form the

“steps” of the DNA ladder

Making Proteins• The order of the four nitrogen

bases (A, T, G, C) along each gene forms a genetic code that specifies what type of protein will be produced

• The production of proteins is called Protein Synthesis

• During this process, the cell uses information from a gene on a chromosome to produce a specific protein

• Protein Synthesis takes place in the ribosomes

The Role of RNA

• The ribosomes are found in the cytoplasm

• The chromosomes are in the nucleus

• How does the information needed to make proteins get from the nucleus to the ribosomes?

• A “messenger” must first carry the genetic code from the DNA (in the nucleus) into the cytoplasm (to the ribosomes)

• The “messenger” is RNA• RNA looks like one side of the

“DNA ladder”• RNA also has four nitrogen

bases: adenine (A), guanine (G), cytosine (C), and uracil (U)

Types of RNA • There are two main types of RNA

• Messenger RNA:– Copies the coded

message from the DNA in the nucleus, and carries the message to the cytoplasm for the ribosomes

• Transfer RNA:– Carries amino acids

and adds them to growing proteins

Protein Synthesis

• Step 1: mRNA Production– In the nucleus, DNA “unzips” between

base pairs, and RNA bases match up along the DNA strands…the genetic info from DNA is transferred to the RNA

• Step 2: mRNA attaches to a Ribosome– mRNA enters cytoplasm, attaches to a

ribosome, production of protein begins• Step 3: tRNA attaches to mRNA

– tRNA carries amino acids to the ribosome, they match up with bases on the mRNA, protein chain grows

• Step 4:Protein Production Complete– Chain grows until the ribosome tells it to

stop…the chain is released and the protein is complete

Mutations• Any change that occurs in

a gene or chromosome is called a mutation

• Mutations can cause a cell to produce an incorrect protein during protein synthesis

• If this happens, an organism’s phenotype (traits) will be different from what it should have been

• If a mutation happens in a body cell, only that cell is affected

• If a mutation happens in a sex cell, offspring can be affected

Effects of Mutations

• Some mutations can be harmful– These mutations can

reduce an organism’s chance of survival

• Some mutations can be helpful– These mutations can

improve an organism’s chance of survival

• Some can be neither